Digital imaging systems (e.g., cameras) have quickly become a standard feature for consumer electronic portable devices including portable multimedia players, smart phones, laptops, and tablet computers. The image quality expected from these portable cameras has grown, as cameras with higher quality optics and higher resolution sensors are being incorporated into such small devices. As portable device dimensions shrink, so do the dimensions of the incorporated camera modules. At such small scales, mass produced camera modules become more susceptible to image quality degradation due to slight deviations and/or contaminations in the optical components introduced during manufacture and imaging system assembly. For example, a camera module's reproduction of color varies as a consequence of variations in its constituent image sensor, lens, and infrared filter. For mass production of high quality camera modules, it is important that the color response is accurate and consistent across camera modules.
In order to achieve accurate and consistent color reproduction, the color response of a camera module may be calibrated during manufacture testing. Calibration usually involves measuring the color response of a camera module under a fixed set of conditions (e.g., a fixed illuminant) and recording those values. The color response values are then compared to those of an “ideal” module, to derive correction factors. The correction factors are then stored with each specimen of the module, and are automatically applied to each picture that is taken during end-user or in-the-field use.
When calibrating the color response, the color response measurements of the camera device under test (DUT) and those of the ideal module should be taken under the same lighting conditions. Ideally the color response of the DUT and the color response of the ideal module are measured while using the same light source. However, this is unrealistic for mass production environments because often times, not all camera modules are calibrated at the same location. For example, in high volume manufacture, the calibration of the specimens of a given camera module design may have to take place at different factories or at different production lines within a factory. Reproducing the exact same lighting conditions each time can be difficult because the power spectrum or spectral distribution of light emitted by even the same brand and model of light source may vary. Some of this variation may be controlled by adjusting all of the light sources to the same color temperature.